The Conservatives face losing their Commons majority if local election results are replicated at a general election, according to a Sky News projection.
Analysis suggests that it would mean a hung parliament with Prime Minister Boris Johnson's Tories remaining the largest party at Westminster but falling 48 seats short of an overall majority.
The projection is based on local election results from 1,700 wards and an analysis of change in vote share since 2018 across 87 local authorities. It predicts that the Conservatives would have 278 seats, just seven more than Labour on 271.
The figures come after Mr Johnson admitted it was a "tough night" for his party after it lost some key local councils in London to Labour - though the opposition party had mixed results outside the capital.
Some local Conservative leaders were blaming the prime minister for shock losses as partygate and the cost of living dominated comments at the doorstep.
As more results came in, the number of Tory council seats lost topped 450.
One southern Tory MP told Sky News: "It's pretty grim. The PM is to blame, no one else, and there are now 19 Tory London MPs who will be baying for blood. The PM is killing our traditional vote."
Separately, David Simmonds, Conservative MP for Ruislip, Northwood and Pinner in northwest London, told Sky News that partygate was the "number one issue I've heard about on the doorstep".
He added: "It's far outstripped issues like the cost of living."
Mr Simmonds said voters were "clearly sending a message to my party, to the government, to say there needs to be some change, we need to show we've got that sense of grip".
But Mr Johnson's allies warned it was not time to change leader as they insisted Labour's gains fell short of what was needed for the party to secure a general election victory.
The prime minister said: "We've had a tough night in some parts of the country, but on the other hand, in other parts of the country, you're still seeing Conservatives going forward and making quite remarkable gains."
However, Labour gains in the rest of England were more modest.
The party was also buffeted by the announcement on Friday that leader Sir Keir Starmer will be investigated by police in Durham over "beergate" - the allegation that he broke lockdown rules while drinking with colleagues in April last year.
The Conservatives were dealt a blow in Somerset where the Lib Dems won a strong majority, taking back control of the council 13 years after losing it to the Tories.
Sir Keir, speaking from Barnet on Friday morning, said the results represented a "massive turning point for the Labour Party".
"From the depths in 2019 we are back on track now for the general election, showing what the change that we've done, the hard change that we've done in the last two years, what a difference it has made," he added.
Emily Thornberry, Labour's shadow attorney general, told Sky News: "We still have a mountain to climb, we're not pretending that we don't.
"We're back - and we're on the right path to becoming the next government."
Labour sources were particularly jubilant about grabbing the Tories' "jewel in the crown" in Wandsworth, the London borough which has been held by the party since 1978.
It described the results as "shattering" for the Conservatives and said they represented a rejection of Mr Johnson.
A spokesperson for the party said: "The question every decent Conservative will be asking themselves is how much further they are willing to fall for a man who never fails to put his own interest above his councillors, his MPs, his party, and his country."
But Iain Duncan Smith, the Conservative MP and former party leader, told Sky News: "No mid-term council election is ever positive in a way... You really can't extrapolate from here."
But he admitted that "London has not been good".
Maidstone, whose MP is Conservative, saw Labour and the Greens pick up one seat each to pull the Tory-run council into no overall control.
While the Conservatives have seen a substantial drop in support in the south of England, Labour has seen a larger drop in the north.
A rise in Lib Dem support across England has seen the party winning Hull from Labour, gaining from the Tories in Merton and in West Oxfordshire, the constituency former prime minister David Cameron previously represented.
Results in the local elections could also prove key to the future of the prime minister - and whether rumblings of backbench discontent escalate into a chorus of opposition triggering a no-confidence vote.
They will also shed light on whether Sir Keir has been able to make ground amid the pressures facing the PM as a result of partygate, the cost of living crisis and questions about the culture in Westminster.